This invention relates to processes for forming fiber reinforced tapes. More particularly, this invention relates to processes for forming impregnated carbon fiber tapes using a crosshead type die.
The use of crosshead dies for coating metallic wires of various types with plastic materials is well known. See, e.g., U.S. Pat. Nos. 4,341,509, 4,247,504, and 4,150,929. Crosshead dies have also been used to coat tire cord, with, for example, an acrilonitrile/butadienne copolymeric rubber. See, U.S. Pat. Nos. 4,328,133 and 4,300,878. Other patents disclosing the use of crosshead dies include U.S. Pat. No. 4,171,195, 4,172,106 and 4,274,821.
U.S. Pat. No. 3,485,912 discloses that "high tensile strength elements, such as tapes" may be formed by
manufacturing a plurality of synthetic resinous oriented strands or filaments and subsequently encapsulating such materials within a relatively soft low tensile strength matrix or casing. PA1 a first thermoplastic or thermosetting plastic material having disposed therein a plurality of reinforcing filaments such as fibers or filaments of glass.
However, this patent does not disclose the composition of the matrix. Likewise, U.S. Pat. No. 3,461,197, discloses a similar article which consists of
U.S. Pat. No. 3,758,247 refers to a crosshead die which is used for coating flat cables utilizing a melt consisting of rubber or plastics including polyethylene or polypropylene. Finally, U.S. Pat. No. 3,993,726, discloses the use of "a crosshead extruder to impregnate the rovings with a thermoplastic polymer under high pressure".
The use of air to spread a carbon fiber prior to impregnation is disclosed in several patents. In U.S. Pat. No. 3,704,485, an apparatus for spreading a graphite fiber tow is disclosed which involves the use of a pulsating, gaseous medium. U.S. Pat. Nos. 3,873,389 and 3,795,944 both disclose the use of a counter air flow means for spreading graphite fiber tows which are to be impregnated with thermosetting materials.
The cooling of impregnated carbon fiber bundles following crosshead die extrusion is also known. For example, in U.S. Pat. No. 3,993,726, cooling sprays or a cooling bath are used, and in U.S. Pat. No. 3,728,424, electrical cables impregnated with a polyester in a crosshead die are cooled by passing through a first ambient air cooling zone and then into a second water cooling tank.
However, none of the above-identified patents discloses either the inventors' specific gas jet spreading system, their crosshead die design or their air cooling means as disclosed hereafter.
In the prior art, carbon fiber-based, prepreg tapes were normally prepared using thermoset resins, with the thermoset resin itself literally painted onto the tapes in an uncured condition. However, care had to be taken to insure that the temperature of the tape remained low during storage, so that autocuring did not occur. When the tape was to be used in forming an article, layers of the tape with the desired orientation were placed on an appropriate form, heated to the curing temperature in a controlled manner and normally under vacuum and/or externally applied pressure baked for the period of time necessary to insure a complete cure. But a major problem with this system was that after curing, it was impossible to reform the article if errors in forming had occurred. Therefore, large amounts of wasted material often resulted. Finally, using this method, it was difficult to obtain thick, uniform coatings and the adhesion of the thermoset polymer to the carbon fiber was often poor.
Thus, it is an object of the instant invention to prepare a simple and inexpensive, but readily usable, crosshead die.
It is another object of this invention to prepare such a die for use in impregnating fibers with thermoplastic, resinous, materials.
It is another object of this invention to prepare tapes which are based upon a thermoplastic resinous, fibrous matrix.
These and other objectives are obtained by utilizing the process of the instant invention.